1. Field of the Invention
The present invention relates to an electronic device, more particularly, to an electronic device comprising a housing of an insulating material having mounted therein an electronic element such as a semiconductor element and a quartz element, and a method of producing the same. The present invention also relates to a package base for mounting an electronic element, i.e. a component part of the electronic device.
2. Description of Related Art
As is well known, various electronic devices currently have been proposed and practically used. Some examples include an IC package having an IC chip mounted thereon and a crystal resonator having a quartz element mounted thereon. These electronic devices have been generally produced by mounting an electronic element in a housing constituted of a package base and a cap, and sealing the housing.
For example, a crystal resonator is a quartz element-mounted package structure of surface mounting type utilizing a quartz element as a reference signal source of a clock or a mobile communication unit, and generally has a structure as shown in FIG. 1. A crystal resonator 60 comprises a quartz element 58, a package base 50 for accommodating and bonding the quartz element 58 in a closed space (cavity) 57, a junction member 55 for bonding the quartz element 58 and the package base 50, a cap 59 for contacting and closing the package base 50 to thereby form the cavity 57, and a sealing member 56 for sealing the package base 50 and the cap 59. The interior of the cavity 57 is maintained under a vacuume condition or charged with an inert gas such as nitrogen for improving the resonation characteristic (Q value) of the quartz element 58 and reducing its variation with time.
FIG. 2 is a cross-sectional view showing the package base 50 for mounting the quartz element, used with the prior art crystal resonator shown in FIG. 1. The package base 50, as shown in FIG. 2, is constituted of a base member 51 for forming a cavity along with a cap arranged in opposed relation thereto, a wiring 54 for electrically connecting the interior and the exterior of the cavity, an internal electrode 52 for fixing and electrically connecting the quartz element, and an external electrode 53 for connecting it to an external circuit. The substrate 51 is generally formed from a ceramic such as alumina. The wiring 54, the internal electrode 52 and the external electrode 53 are each formed by baking a conductive metal such as W, Mo, AgPd alloy or the like on the substrate member 51, and suitably plated with Ni or Au.
In the prior method of producing the crystal resonator, a conductive adhesive has been used widely as a junction member for bonding the quartz element and the package base. However, since a heat curing process is required for the adhesive, the adhesive develops a distortion due to its shrinkage, which in turn causes stress to be exerted on the quartz element. As a result, the Q value of the quartz element is varied, thereby causing the problem of deterioration of the stability of its characteristics.
Recently, there has been a tendency for a hard metal having little variation with time to be used as a junction member for the crystal resonator, to satisfy the requirements of long lasting stability of the characteristics along with an improved Q value of the quartz element. As explained below with reference to FIG. 3, for example, a method has been proposed to use a AuSn alloy paste as a junction member.
In the method shown in FIG. 3, the first step is to coat a AuSn alloy paste 61 on a predetermined portion of an internal electrode 52 formed on a substrate 51 of a package base 50, and then apply the quartz element 58 thereon so that they contact. The AuSn alloy paste 61 is composed of spherical particles of the AuSn alloy containing 29 atomic % of Sn dispersed in a flux. Since the AuSn alloy paste 61 is viscose, the quartz element 58 is temporarily tacked on the internal electrode 52 of the substrate 51. Thereafter, the AuSn alloy paste 61 is melted by heating it to at least its melting point of 278xc2x0 C., thereby to bond the quartz element 58 and the substrate 51. After bonding, in order to remove the flux, the assembly is washed and dried, after which it is covered with a cap (not shown) and a vacuum is applied in a vacuum tank to obtain a sealed product. However, due to the narrow gap between the quartz element and the package base (substrate) after bonding, use of the AuSn alloy in this method makes it difficult to remove the flux by washing, and undesirably leaves a residue of flux on the quartz element. The residual flux can be the cause of the above variation with time.
Further, Japanese Unexamined Patent Publication (Kokai) No. 2000-232332, for example, discloses a method of ultrasonic bonding of a quartz element with the package base by forming an Au bump on an electrode of the quartz element, as explained below with reference to FIG. 4.
In the method shown in FIG. 4, the first step is to form an Au bump 62 on the electrode (not shown) of the quartz element 58, using a ball bonder. Then, the quartz element 58 is brought into contact with the package base 50 at a predetermined position (the internal electrode 52 on the substrate 51) as shown, and an ultrasonic welder 63 for applying the ultrasonic vibration from above the quartz element 58 is applied to the quartz element 58. The quartz element 58 is moved horizontally by ultrasonic waves under application of a load. The Au bump 62 and the internal electrode 52 of the package base 50 are bonded to each other by the friction generated therebetween by the ultrasonic waves. The ultrasonic junction occurs only in such a range that the Au bump 62 is in contact with the substrate 51 of the package base 50, and therefore the junction area is limited. After complete ultrasonic bonding, a cap is applied to the assembly, which is then vacuum sealed in a vacuum tank. This method, however, suffers from the problem that the junction area is so small that insufficient bonding strength results in low shock resistance. Further, when the load for ultrasonic bonding is increased in order to increase the bonding strength, a new problem of breaking or cracking in the quartz element is caused.
An object of the present invention is to solve the prior art problems caused using a conductive adhesive, an AuSn alloy paste, an Au bump or the like as a junction member for the electronic element in the production of an electronic device such as a crystal resonator, thereby providing an easy-to-produce electronic device having stable characteristics exhibiting little variation with time and high shock resistance, and a method of producing such a device.
Another object of the present invention is to provide a crystal resonator or package structure of a quartz element which can be produced easily, and which has stable characteristics with little variation with time and high shock resistance.
Still another object of the present invention is to provide a package base for mounting an electronic element that can be advantageously used for the production of the electronic device according to the invention.
These and other objects of the present invention will be easily understood from the following detailed description of the present invention.
According to one aspect of the present invention, there is provided a package base for mounting an electronic element, comprising an insulative substrate and an internal electrode formed at a predetermined position on the substrate, the internal electrode being bonded to the electronic element, in which a junction member comprising a junction layer composed of a junction alloy having a melting point higher than the mounting temperature and an anti-oxidation conductive layer covering the junction layer are provided on the internal electrode.
According to another aspect of the present invention, there is provided an electronic device comprising a housing which comprises a package base and a cap covering the surface of the package base, the housing containing at least one electronic element mounted therein, in which the package base comprises an insulative substrate and an internal electrode formed at a predetermined position on the substrate, the internal electrode being bonded to the electronic element, and the electronic element and the internal electrode are connected electrically to each other, through a junction member comprising a junction layer formed on the internal electrode and composed of a conductive junction alloy having a melting point higher than the mounting temperature and an anti-oxidation conductive layer covering the junction layer.
According to still another aspect of the invention, there is provided a method of producing an electronic device comprising a housing which comprises a package base and a cap covering the surface of the package base, the housing containing at least one electronic element mounted therein, in which the method comprises the steps of:
forming an internal electrode for bonding the electronic element at a predetermined position on the substrate of the package base;
sequentially forming, on the internal electrode, a junction layer composed of a conductive junction alloy having a melting point higher than the mounting temperature and an anti-oxidation conductive layer covering the junction layer to thereby form a junction member; and
bonding and electrically connecting the electronic element and the internal electrode to each other through the junction member.
Now, the terms used in this specification will be explained. The term xe2x80x9celectronic elementxe2x80x9d is defined in a broad sense, and includes, for example, a semiconductor element normally mounted on an electronic device, such as an IC chip, a LSI chip or a VLSI chip, a part such as a capacitor, a resistor or a conductor, and a quartz element. Thus, the term xe2x80x9celectronic devicexe2x80x9d as used herein is defined as any of various devices employing an electronic element, such as a semiconductor device and a crystal resonator. In other words, as long as the present invention is effectively applicable, the electronic device and the electronic element mounted thereon is not specifically limited. In the practice of the present invention, the quartz element and hence the crystal resonator are especially useful.